912ul valve sticking.

Hi Ron,

Having had a valve problem on a 912 I can tell you that it definitely doesn't run smoothly at any time. It runs so rough that you want to get back on the ground pretty quick. If I leave my engine for a while and then hand prop it you can feel that the compression is uneven, this is normal. It you try it after the engine has been warmed up it feels tight on the compression strokes.

If making the mixture richer improves the running, then have you got an air leak somewhere? The carb rubbers are the usual suspects.

If you have a cylinder problem, then the state of the spark plugs will give you a clue. If you have sooty plugs on both cylinder on one side, then you probably have a fuel/air problem on that side. If it is a single cylinder, then it could be a valve problem. Have you got the kit to do a cylinder leak down test? If you have an open valve, you'll hear the air escaping back through the carb.

I've been down the flushing oil route in the past and no good came of it. you have to find the root cause and fix it. In my case it was a bizarrely worn inlet valve that had to be replaced (relatively straightforward).

Hi Ron,

Start with and do the cheapest and most common solution first before jumping into something more invasive and costly.

If the engine is older and the carbs haven't been overhauled for a long time then this may be in your very near future too. 

Since the plane has been sitting for sometime clean your idle jet out. The choke shouldn't have any effect on a sticky valve to make it run smoother. Pop the bowl off. It just takes a straight blade screwdriver and it's right next to the main jet. Use a piece of wire to rod it out. DO not put anything like a drill bit into the hole and make it larger. You can strip back the insulation of a piece of 16 ga. wire about 4" - 5" and then unwined ONE strand from that and place it through the center hole of the idle jet. Roll the idle jet and move it back and forth on the wire to remove any debris. Then follow that with some carb cleaner spray and then some high pressure air. Then replace the jet. I see this fairly often from aircraft that sit a lot. This cleaning works like a charm on an idle jet and will not damage it. I've been doing it for 20 years.

Hi guys. 
Thank you for your input. I rebuilt the carbs about 50 ish hours ago. I have already pulled the carbs so I will follow the path of least resistance and clean and reinstall. 
I still don't understand though why one carb was Carboned up black. Could it be because carb unbalance caused backfiring? 
still confused,

Ron

Ron,

Perhaps the black in the carb throat is unrelated. Do you have an exhaust leak directly beneath that carb?

Not saying you may not have a valve not fully closing, but try the simpilist and most common things first. If you truley do have a stking valve then that means pulling the head, sending it in to a service center and that will cost you a lot more time and money. Try the carbs first. Easy and inexpensive.

If one cab seems have have a really dirty throat over the other. Do you have K&N air filters? If you do could one be over oiled? Just trying to get more info surronding your problem and possible simple causes.